For the noncontact photogrammetric coordinate measurement on the surface of an object in a close range, the dimensions of the object are derived from images which reproduce the object from various perspectives, by transformation of the image data into an object coordinate system. For this purpose, the image data are processed in a data processing unit. The basis of the coordinate calculation is the determination of the camera orientations of the images involved.
As know from the prior art, it is possible to record those area sections of the object surface which are to be surveyed at different times from different perspectives by means of a single camera and then to process the two-dimensional image data in each case by means of an image processing system to give a so-called three-dimensional image. In each case depth information is coordinated with the pixels of the three-dimensional image, so that 3D image coordinates in an image coordinate system which is determined from the cameras and the perspectives thereof are coordinated with each pixel to be investigated, in particular all pixels. Different image processing methods for producing such a 3-dimensional image from a plurality of 2-dimensional images showing the same scene from different perspectives are disclosed in the prior art.
It is furthermore possible, as likewise known from the prior art, to carry out a substantially simultaneous recording with the aid of a plurality of cameras in stead of the recording of the area section from different perspectives at different times by means of one camera. This has the advantage that both a three-dimensional determination of the area section is possible without camera movement and determination of the respective camera orientations is dispensed with since the cameras can have a fixed relative orientation and distance to one another.
The prior art discloses different 3D image recording devices which are substantially composed of two or three cameras which are housed a distance apart, i.e. having a stereoscopic basis, in a common housing for recording a scene from in each case different but fixed relative perspectives. Since the recorded area section does not inevitably have characteristic image features which permit electronic processing of the images, markings can be applied to the area section. These markings can be produced by means of a structured light beam, in particular laser beam, projecting from the 3D image recording unit onto the area section, which, for example, projects an optical screen or an optical marking cross. Usually, such 3D image recording units also comprise an image processing device which derives a three-dimensional image from the plurality of images from different perspectives which are recorded substantially simultaneously.
Such 3D image recording units are, for example, the image recording systems from “CogniTens”, which are known by the trade names “Optigo” and “OptiCell” and contain three cameras arranged in an equilateral triangle, and the system “Advent” from “ActiCM” with two high-resolution CCD cameras arranged side by side and a projector for projecting structured light onto the section to be recorded.
The determination of the coordinates of recorded image elements to be surveyed is effected as a rule by means of referenced markings within the image, from which markings the actual 3D coordinate measurement takes place. Here, the image coordinate system which relates to the recorded three-dimensional image and is therefore based on the 3D image recording unit is transformed into the object coordinate system within which the object to be surveyed exists and which is based, for example, on the CAD model of the object. The transformation takes place on the basis of recorded markings whose positions in the object coordinate system are known. Here, accuracies of less than 0.5 millimeter are achieved with the 3D image recording units known from the prior art.
3D scanning systems, in particular in the form of 3D laser scanners, which carry out deep scanning within a sector and produce a point cloud are furthermore known.
Here, a distinction should be made between serial systems in which a laser beam scans an area line by line, parallel systems in which the scanning line is fanned out over an area and completely parallel systems, so-called RIMs or range imaging systems which simultaneously scan a multiplicity of points within a sector and thus carry out a deep recording of the sector. Common to all these systems as a rule is that the deep scanning is effected by means of at least one distance-measuring laser beam which in particular is moved over the area. In particular, such serial systems are widely used and are commercially available, for example, under the product designations “Leica HDS 6000”, “Leica ScanStation 2”, “Trimble GX 3D Scanner”, “Zoller+Fröhlich IMAGER 5003” and “Zoller+Fröhlich IMAGER 5006”.
One problem of every 3D image recording unit is that the recording region within which an image recording can be made with the required resolution is limited by the design. In the case of the three-dimensional determination of relatively large objects, the making of a plurality of individual three-dimensional recordings from different positions and orientations of the 3D image recording unit is therefore unavoidable. This multiplicity of smaller image recordings is subsequently combined by means of matching of overlapping image regions with the aid of markings within the recorded sector to give a larger three-dimensional total image. Different methods for achieving this object are disclosed in the prior art. A general problem in the case of these methods is that the individual three-dimensional images which are to be combined to give a larger image must have an overlap region. The discrete changing of the position of the 3D image recording unit from a first area section having at least one reference point to a second area section which is a distance away from the first area section and contains no reference point is not possible by means of the image-processing systems unless further images which connect the two area sections were recorded. It is therefore necessary to carry out a multiplicity of intermediate image recordings in order optically to connect the two area sections a distance apart which are to be surveyed and to permit cohesive image processing. By the recording of a multiplicity of three-dimensional images which have no direct measuring content, the entire measuring method is slowed down and memory and computational resources are consumed. Furthermore, the coordinate measurements within the image recording which are inevitably associated with small measurement errors have a drastic effect on the accuracy of measurement on combination of the multiplicity of images, in particular in the case of remote reference points.
Furthermore, measuring systems and methods in which the 3D image recording unit is supported by the head of an industrial robot or a portal coordinate-measuring machine and is adjustable are further known from the prior art. Owing to the high weight of a high-quality and high-resolution 3D image recording unit, which in some cases is more than 10 kilograms, a precise determination of the position of the 3D image recording unit with the required accuracy which matches the image-recording accuracy is not possible since this would require such a stable structure of the handling system that the field of use of the 3D image recording unit would be limited to stationary systems. For example, use in the interior of vehicle bodywork would be complicated or made impossible thereby. Owing to their low accuracy of measurement, which is considerably less than that of a precise 3D image recording unit, industrial robots are unsuitable for external referencing. Once again, portal coordinate-measuring machines are not designed for supporting heavy loads and, under high mechanical load, do not give measured results which can be used for referencing. For this reason, any measured position values which are delivered by the handling system and could provide information about the absolute and/or relative position of the 3D image recording unit cannot be used for referencing the image recordings, in particular a plurality of three-dimensional image recordings of different, non-cohesive area sections.